The grain threshing, separating and cleaning assemblies of a combine harvester operate at maximum efficiency only if there is a substantially constant crop material feed rate. A change in the feed rate of grain, or the feed rate of crop material other than grain affects the operation of the threshing, separating and cleaning assemblies. The standard adjustments found on harvesting machines make it possible to accommodate and harvest a variety of crops under a variety of crop and weather conditions. However, it takes time and skill to make these adjustments. A change in the quantity of material passing through a combine harvester can substantially change the efficiency of the harvesting operation even if the weather and crop conditions remain unchanged.
A decrease in the quantity of crop material passing through the threshing cylinder or rotor and concave can result in a substantial increase in the quantity of cracked grain. Cracked grain is blown out of the harvester and on to the ground by the cleaning system fan.
An increase in the quantity of crop material passing through a harvester can overload the cleaning system, form a mat of crop material on the sieve of the chaffer and a large portion of the threshed grain will end up on the ground rather than in the grain tank. An increase in the quantity of crop material passing through a harvester can also overload the separating assemblies and cause grain loss.
Combine harvesters are designed to handle close to the maximum capacity of the threshing assembly, separating assembly and cleaning assembly, and the power source at any given time. To do this, the capacity of each of the assemblies and the power source must be balanced with each other. Due to variations in crop conditions and weather conditions, it is not possible to attain a perfect balance between the threshing, separating and cleaning assemblies and the power source. In practice operators generally adjust the threshing, separating and cleaning assemblies for a crop material feed rate which leaves sufficient power available to propel the combine with the current field conditions and to unload the grain tank. The operator then varies the grounds speed of the harvester as best he can to maintain a constant crop material feed rate. Experienced combine operators can maintain crop material feed rates within a range that attains satisfactory threshing, separating and cleaning. However, due to operator fatigue, even experienced operators fail to maintain crop material feed rates within an acceptable range after several hours of operation without a rest period.
Systems that maintain a constant load on a harvester internal combustion engine by controlling harvester ground speed have been tried. Some of these systems sense engine load through the engine governor and adjust harvester ground speed to maintain engine load within a narrow range. When a field to be harvested is dry and flat, the engine load control can maintain a fairly constant crop material feed rate. Hills or slopes in a field can vary the power required to propel a combine harvester through the field to such an extent that an engine load based control system is rendered unacceptable for controlling crop material feed rate.
A field with hard, compact soil in some areas and soft soils in other areas will have variable power requirements for propelling a combine harvester through the field. This variation in the total engine load can result in an engine load based control system making large changes in ground speed and unacceptable changes in crop material feed rates.
Attempts have been made to measure the load on combine harvester threshing cylinders and employ the measured load to control crop material feed rates. Measurement of the load on the threshing cylinder has been difficult. A system based on the creep of a cylinder drive belt in the driven and driving pulleys or sheaves worked well. However, if the cylinder drive belt becomes over loaded and actually slips, the system fails to control crop material feed rate. For a belt drive based system to measure threshing cylinder load to control crop material feed rate, the belt drive must be capable of transmitting more torque than will be required, to avoid belt slip. Such belt drives are expensive and are generally not available on high-capacity combine threshing cylinder drives.